In order to reduce radio wave noise generated by ignition, a spark plug that includes a resistor inside of a through hole of an insulator is known (for example, see JP-A-2009-245716). In this spark plug, a conductive seal is disposed between the resistor and a center electrode. A contact portion between the resistor and the conductive seal is formed in a bowl shape that projects toward a tip end side around the central axis of the through hole. Consequently, this expands the contact portion between the conductive seal and the resistor compared with the case where the contact portion lies in a horizontal plane. This reduces sealing failure (such as peeling) between the conductive seal and the resistor.
However, in the above-described technique, the resistor has a shorter effective length compared with the case where the contact portion lies in a horizontal plane. This may decrease radio-wave noise reduction performance.
The main advantage of the present invention is a technique that reduces sealing failure between the conductive seal and the resistor while suppressing decrease in radio-wave noise reduction performance.